A conventional refrigerant circuit includes compressor 1, condenser 2, expansion valve 3, evaporator 4, and accumulator 5 connected serially as shown in FIG. 1. Evaporator 4 is provided with fan 6 having motor 6a which rotates fan 6. The air flow volume of fan 6 is varied by transfer switch 7.
FIG. 2 shows the construction of transfer switch 7. Transfer switch 7 includes movable contact point 71, fixed contact point 72 which is connected to electric source 8, and fixed contact points 73, 74, and 75, which are each connected to motor 6a of fan 6. Fixed contact point 73 is coupled to motor 6a through resistors R1 and R2 which are connected in series. Fixed contact point 74 is coupled to motor 6a through resistor R2. Fixed contact point 75 is connected directly to motor 6a. Movable contact point 71 moves on fixed contact points 72, 73, 74, and 75 to vary the fan speed. When fixed contact point 73 is coupled to fixed contact point 72 through movable contact point 71, because the resistance between motor 6a and fixed contact point 73 is large, the electric current flowing to motor 6a is small and motor 6a rotates at its lowest speed. Accordingly, the air flow volume of fan 6 is the smallest. When fixed contact point 74 is coupled to fixed contact point 72 through movable contact point 71, the resistance between motor 6a and fixed contact point 73 is smaller. The amount of electric current increases and motor 6a rotates at a higher rotational speed. Accordingly, the air flow volume of fan 6 also increases. When fixed contact point 75 is coupled to fixed contact point 72 through movable contact point 71, the resistance between motor 6a and fixed contact point 75 is at a minimum. The amount of electric current is largest, motor 6a rotates at its highest speed, and the air flow volume of fan 6 is a maximum.
Expansion valve 3, shown in FIG. 3, has orifice 33 having a uniform cross-sectional area, high pressure side 31, and low pressure side 32. The cross-sectional area of orifice 33 must be large enough to accommodate the refrigerant flow volume when the air conditioning load is greatest. Accordingly, when the air flow volume of fan 6 is reduced and evaporator 4 operates at a reduced capacity, excess refrigerant accumulates at low pressure side 32 of valve 3 in the refrigerant circuit. To prevent this, the volume of accumulator 5 must be increased to house excess refrigerant and to prevent excess refrigerant from remaining at low pressure side 32 of valve 3. However, if the volume of accumulator 5 is increased, the space for housing accumulator 5 must be increased. This increases the space in the engine compartment required for the refrigeration circuit and is highly disadvantageous as such increased space may not be available.
Another cnoventional expansion valve is disclosed in allowed Japanese Utility Model No. 59-21,938. The cross-sectional area of the opening of this expansion valve can be varied by using the negative pressure of the engine. However, this expansion valve has a complicated construction and is expensive to produce.